The present invention relates generally to wireless computer networking techniques. More particularly, the invention provides a method and a system for estimating physical location of wireless transmitters in wireless networks according to a specific embodiment. Merely by way of example, the invention has been applied to a computer networking environment based upon the IEEE 802.11 family of standards, commonly called “WiFi.” But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to Ultra Wide Band (“UWB”), IEEE 802.16 commonly known as “WiMAX”, cellular wireless networks such as CDMA, GSM, GPRS, and others.
Computer systems proliferated from academic and specialized science applications to day to day business, commerce, information distribution and home applications. Such systems include personal computers, which are often called “PCs” for short, to large mainframe and server class computers. Powerful mainframe and server class computers run specialized applications for banks, small and large companies, e-commerce vendors and governments. Smaller personal computers can be found in many if not all offices, homes, and even local coffee shops. These computers interconnect with each other through computer communication networks based on packet switching technology such as the Internet protocol or IP. The computer systems located within a specific local geographic area such as office, home or other indoor and outdoor premises interconnect using a Local Area Network, commonly called, LAN. Ethernet is by far the most popular networking technology for LANs. The LANs interconnect with each other using a Wide Area Network called “WAN” such as the famous Internet.
While conventional computer networks proliferated in the past, currently wireless communication technologies are increasing in popularity. That is, wireless communication technologies wirelessly connect users to the computer networks. Examples of wireless networks include, but not limited to, wireless local area networks (WLAN) and cellular networks. One desirable application of wireless networks is to provide wireless access to the LAN in the office, home, public hot-spots, and other geographical locations. As merely an example, the IEEE 802.11 family of standards, commonly called WiFi, is the common standard for such wireless application. Among WiFi, the 802.11b standard-based WiFi often operates at 2.4 GHz unlicensed radio frequency spectrum and offers wireless connectivity at speeds up to 11 Mbps. The 802.11 g compliant WiFi offers even faster connectivity at about 54 Mbps and operates at 2.4 GHz unlicensed radio frequency spectrum. The 802.11a provides speeds up to 54 Mbps operating in the 5 GHz unlicensed radio frequency spectrum. The WiFi enables a quick and effective way of providing wireless extension to the existing LAN.
In order to provide wireless extension of the LAN using WiFi, one or more WiFi access points (APs) connect to the LAN connection ports either directly or through intermediate equipment such as WiFi switch. A user now wirelessly connects to the LAN using a device equipped with WiFi radio, commonly called wireless station that communicates with the AP. The connection is free from cable and other physical encumbrances and allows the user to “Surf the Web”, check e-mail or use enterprise and e-commerce computer applications in an easy and efficient manner. Since access points have limited signal coverage, multiple access points may be required to provide signal coverage throughout a facility. Another advantage of wireless networks is that wireless stations can move throughout the signal coverage area and continue to connect to the computer network. In a typical operational wireless network, multiple wireless stations operate from different locations within the geographic area comprising the wireless network. Station locations may also change from time to time as the users carrying those devices move.
Determining physical location of a wireless station, or that of a signal transmitting device, is useful for many applications, such as dispatching and location based applications. Physical location information regarding a wireless station is also useful if said station either deliberately or unknowingly is causing harm to the wireless network in the form of security breach, intrusion, denial of service attack (DOS), and the like. Knowing the physical location of the wireless station in these scenarios enables the network administrator to track down the station and take action (e.g. remove it from the vicinity of the network) to avoid security or performance catastrophe. However, location tracking capability is often not supported by standard WiFi communication systems and hence a solution is highly desirable.
Prior solutions have attempted to provide mechanisms to determine physical location of a wireless station, with varying degrees of success. One technique known in prior art is to analyze time delays encountered by the radio frequency signals received by (e.g., as in global positioning system or GPS) or transmitted from (e.g., as in time difference of arrival or TDOA used in cellular network) the station with respect to transmitters (e.g., satellites in GPS) or receivers (e.g., base stations in cellular network) located at known reference positions. However this approach has several limitations in local area wireless networking environment. For example, the GPS is often infeasible for indoor environments (e.g., office buildings, warehouses, apartments, commercial facilities, and like) as the signal from satellites may not reach indoors. Further, in these environments, the time delay encountered by the radio frequency signal between the transmitter and the receiver is usually too small to be accurately measured. Also these techniques require specialized hardware and/or software in the wireless station which is often not available.
Another conventional technique involves triangulating the position of the wireless station with respect to at least three reference locations, based on signal strength measurement. The receive signal strengths at different wireless access points from the transmitting wireless station are measured. The station is then estimated to be located at the center of the triangle formed by three access points for which a cost function of the receive signal strengths (e.g., sum of squares of received signal strengths at three access points) is maximized. However, this technique fails to account for signal attenuation and reflections resulting from spatial layout components such as walls, doors, partitions, obstacles and like, and hence the location estimate is often inaccurate.
Yet another conventional technique involves definition of areas, sometimes called “locales”, within the geographic region of interest. The signal strength signature is then created for each area representing received signal strength at one or more network access points from the device transmitting from the area. During the operation, the area in which a transmitting wireless station resides is determined by comparing with the signal strength signature for areas, the observed signal strengths from the transmitting wireless station received at the network access points. The comparison is done via pattern matching. By way of limitation, this deterministic pattern matching approaches, fails to compute and represent the uncertainties in the location of the wireless station arising out of factors such as antenna orientation of wireless station, transmit power variations, imprecise knowledge of obstacles, movement of people and obstacles (e.g. opening/closing of doors) within the area of interest, and like. Not representing the reality of environment can be a serious limitation, especially in applications directed to security. Additionally, some of these techniques require prior definition of areas which can be a cumbersome procedure requiring manual effort and guesswork. The techniques may also require extensive on-site measurements to come up with the signal strength signatures. This is tedious process requiring manual effort and needs to be repeated for every change in the environment such as changing location of access points, changing area layout, and so on.
Accordingly, there is need for techniques for accurate and realistic estimation of transmitter location in wireless networks requiring less manual effort.